Internal combustion (IC) engines are typically operated using spark ignition or compression ignition. With spark ignition, a mixture of fuel and air is introduced into a combustion cylinder and compressed. A spark plug initiates combustion through the creation of an open spark sufficient to ignite the air and fuel mixture in the cylinder. With compression ignition, fuel is injected into the combustion chamber and the heat generated during compression causes the fuel and air mixture to ignite.
Increasingly restrictive engine emission standards have caused more efficient engine operation and reduced emissions from such engines. Homogenous charge compression ignition (HCCI) may result in significant emission reductions. This same process is also known by the names PCI, PCCI and CAI, which stand for Premixed Compression Ignition, Premixed Charge Compression Ignition, and Controlled Auto-Ignition, respectively. In an engine operating under HCCI, the fuel is introduced into the cylinder earlier in the compression cycle than is typical. The air and fuel are intimately mixed, typically at a high air/fuel ratio or with considerable exhaust gas recirculation (EGR), before compression in the combustion cylinder. As compression occurs, the air temperature increases, and ultimately combustion is initiated at numerous locations throughout the cylinder, as the fuel droplets auto-ignite from the heat of the surrounding air. Typically, combustion occurs at lower temperatures leading to reduced noxious oxides (NOx) emissions.
The use of HCCI has apparent benefits in substantial reduction of NOx emissions. However, difficulties have been encountered in implementing HCCI. Fuel preparation is important for peak operating performance of an HCCI engine. The air/fuel mixture must be intimately and thoroughly mixed. Preferably, fuel breakup occurs early in the compression cycle, allowing for intimate mixture of the air and fuel. It is desirable to create droplets of fuel as small as possible in a combustion cylinder operating under HCCI concepts. High pressure injection of the fuel can be used to create surface instabilities on the fuel droplets, causing the fuel spray to breakup and disperse.
In addition to the problem of control of HCCI combustion, there is the problem of starting the engine and warming it up to arrive at a state with stable HCCI combustion. With a cold engine, HCCI combustion is very difficult to achieve and would require heating the air, obtaining very high compression ratio, or using a very easily ignited fuel or additive. The starting and warm up of an engine (whether diesel, spark-ignition, or HCCI) is affected by the ease with which the fuel can be ignited and its volatility.